Dual alloy turbine wheels

Dual alloy turbine wheels or dual-property turbine disks have some limited use at the present time and are extremely attractive for future use in high performance commercial aircraft engine design. Single alloy turbine disks which are used predominantly in current technology commercial aircraft engines, are forged from vacuum melted ingots or are consolidated by various means from pre-alloyed powders. Such a single alloy must satisfy requirements in both the hub and the rim areas of the turbine disk which requirements are sometimes in conflict. The two extremes in single-alloy turbine engine disks today are the forged disks used in commercial and general aviation turbofan engines and the cast integral turbine wheel typically used in small turbo-prop/turbo-shaft engines and auxiliary power units. The forged alloys used today will typically have superior tensile and low cycle fatigue (LCF) properties, but quite limited creep rupture strength, while the cast wheel alloys will have the reversed properties, i.e. excellent creep rupture strength but relatively poor tensile and LCF properties. Modern turbofan engines, developing a thrust from 3,000 to 55,000 pounds and having cooled separately bladed turbine disks, require a turbine disk hub having maximized tensile strength in order to provide a satisfactory burst margin. The hub area must also have maximized resistance to low cycle fatigue (LCF) cracking and crack propagation in order to ensure long turbine disk life. The hub area must also have good notch ductility to minimize the harmful effects of stress concentrations, either inherent in the design or induced by undetected flaws in critical regions. In general, all the desirable qualities for disk hubs are associated with tough, fine-grained, highly-alloyed materials. In contrast to the hub, tensile stress levels are lower in the ring or rim of a well designed turbine disk, but operating temperatures are higher and creep resistance becomes an important consideration. With the current single alloy disk design philosophy, used for modern commercial aircraft and general aviation engines, the material is chosen primarily to satisfy hub requirements and sufficient cooling air is supplied to the rim to lower its temperature to the level, typically about 600°-700° C., where creep strength of the material is not limiting. If temperatures and stresses rise to levels where creep strength becomes limiting in the rims, large-grained alloys with adequate creep-resistance are employed, but the wheel size and weight are increased, since the large-grained creep-resistance micro structures have inferior tensile properties to fine-grained material.

Hence, from the above it is readily apparent that a dual property turbine disk becomes quite attractive as optimum properties in each area of the disk will allow the cooling air requirements for the disk to be minimized or eliminated, with resulting improvements in engine-operating efficiency. In addition, lighter weight turbine disks, would be possible with a favorable impact on total aircraft performance.

A dual alloy turbine disk which provides optimum properties for both the rim and the hub locations, will also permit superior low cycle fatigue cracking resistance in each area and will contribute to long life components that will reduce repair costs.

The dual alloy turbine disk concept is desirable for both separately bladed disk designs and also integrally-bladed turbine stages as used in small aircraft engines, which are currently made from a single piece casting. These small gas turbine engines are presently used in executive and business jet turboprop applications but are also receiving consideration for replacement of the current reciprocating engines used in the general aviation market.

DISCUSSION OF THE PRIOR ART

The dual property turbine disk concept has 2 major variations--the first involving the use of a single alloy processed differently in the hub and the rim areas. For example, some manufacturers overspeed disks sufficiently to cause plastic flow in the hub which pre-stresses the hub in compression, thus reducing its tensile stresses in normal service. The second major variation of the dual property turbine disk is the dual alloy turbine wheel which utilizes two distinct alloys with dissimilar properties as required for the rim and the hub areas, with an adequate and reliable process to join the dissimilar alloys. The dual alloy turbine wheel concept has been used in the 1950's in connection with military engines which utilized AISI Type 4340 alloy steel hubs fusion welded to Timken 16-25-6 warm-worked stainless steel rims. This particular combination was used because the alloys could be fusion-welded to yield joints of adequate strength and freedom from defects which performed well in service. The advent of stronger alloys, however, made the fusion-welding approach obsolete as the more complex alloys could not be fusion welded in typical disk thicknesses without cracking.

Dissimilar metals may also be welded by the inertia-welding process and this process has found use in the joining of axial-flow compressor disks into spools and in the joining of dissimilar metal shafts and turbine wheels. However, the inertia-welding process has an inherent size limitation in that the largest existing inertia welding machines are only capable of welding joints in nickel-base alloys which are a few square inches in cross section. Hence, this limitation prevents the use of the process in all but the smallest turbine disks.